Fuel cells are used as an electrical power source in many applications. In particular, fuel cells are proposed for use in automobiles to replace internal combustion engines. A commonly used fuel cell design uses a solid polymer electrolyte (“SPE”) membrane or proton exchange membrane (“PEM”), to provide ion transport between the anode and cathode.
Fuel cells in general are an electrochemical device that converts the chemical energy of a fuel (hydrogen, methanol, etc.) and an oxidant (air or pure oxygen) in the presence of a catalyst into electricity, heat and water. Fuel cells produce clean energy throughout the electrochemical conversion of the fuel. Therefore, they are environmentally friendly because of the zero or very low emissions. Moreover, fuel cells are high power generating system from a few watts to hundreds of kilowatt with efficiencies much higher than a conventional internal combustion engine. Fuel cells also have low noise production because of few moving parts.
In proton exchange membrane type fuel cells, hydrogen is supplied to the anode as fuel and oxygen is supplied to the cathode as the oxidant. The oxygen can either be in pure form (O2) or air (a mixture of O2 and N2). PEM fuel cells typically have a membrane electrode assembly (“MEA”) in which a solid polymer membrane has an anode catalyst on one face, and a cathode catalyst on the opposite face. The anode and cathode layers of a typical PEM fuel cell are formed of porous conductive materials, such as woven graphite, graphitized sheets, or carbon paper to enable the fuel to disperse over the surface of the membrane facing the fuel supply electrode, Each electrode has finely divided catalyst particles (for example, platinum particles), supported on carbon particles, to promote oxidation of hydrogen at the anode and reduction of oxygen at the cathode. Protons flow from the anode through the ionically conductive polymer membrane to the cathode where they combine with oxygen to form water, which is discharged from the cell. The MEA is sandwiched between a pair of porous gas diffusion layers (“GDL”), which in turn are sandwiched between a pair of non-porous, electrically conductive elements or plates (i.e., flow field plates). The plates function as current collectors for the anode and the cathode, and contain appropriate channels and openings formed therein for distributing the fuel cell's gaseous reactants over the surface of respective anode and cathode catalysts. In order to produce electricity efficiently, the polymer electrolyte membrane of a PEM fuel cell must be thin, chemically stable, proton transmissive, non-electrically conductive and gas impermeable. In typical applications, fuel cells are provided in arrays of many individual fuel cell stacks in order to provide high levels of electrical power.
The electrically conductive plates currently used in fuel cells provide a number of opportunities for improving fuel cell performance, For example, these metallic plates typically include a passive oxide film on their surfaces wherein the electrically conductive coatings should be thin enough to minimize the contact resistance. Such electrically conductive coatings include gold and polymeric carbon coatings. The electrically conductive coating is applied to bipolar plates in a fuel cell in order to reduce or prevent corrosion during operation.
However, in order to have coatings which best adhere to the stainless steel substrate, the native oxide layer must be removed from the stainless steel substrate prior to coating the substrate. In a chemical vacuum deposition process, various coatings are applied to the stainless steel substrates in a single CVD chamber. However, current methods remove the oxide layer on stainless steel outside of the chemical vapor deposition via an acid bath and the like. Unfortunately, as soon as the stainless steel substrate is exposed to air after the acid bath, an oxide layer forms on the stainless steel substrate. Therefore, current methods which implement acid baths to clean the stainless steel substrate require that the stainless steel substrate be exposed to air prior to insertion into the CVD chamber thereby causing an oxide film to form.
Accordingly, there is a need for a manufacturing method and system to remove and coat the native oxide layer from the stainless steel substrate in a manner which does not expose the stainless steel substrate to air after oxide layer removal and prior to coating the stainless steel substrate.